Spectrometers are optical instruments for measuring the absorption of light by chemical or biological materials. Spectrometers typically generate a plot of absorption versus wavelength or frequency, and the patterns produced are used to identify the substance(s) present, and their internal structure. Spectrometers generally comprise a source of electromagnetic radiation generally referred to as a light source, a wavelength selector (monochromator), a sample, a detector, a signal processor, and a readout.
Most spectroscopic methods can be differentiated as being either atomic or molecular based on whether or not they apply to atoms or molecules. Along with that distinction, spectroscopic methods can be classified based on the nature of their interaction. Absorption spectroscopy uses the range of the electromagnetic spectra in which a substance absorbs. This includes atomic absorption spectroscopy and various molecular techniques, such as infrared spectroscopy and Raman spectroscopy, in that region and nuclear magnetic resonance (NMR) spectroscopy in the radio region. Emission spectroscopy uses the range of electromagnetic spectra in which a substance radiates (i.e. emits). The substance first must absorb energy. This energy can be from a variety of sources, which determines the name of the subsequent emission, such as luminescence. Molecular luminescence techniques include spectrofluorimetry.
Scattering spectroscopy measures the amount of light that a substance scatters at certain wavelengths, incident angles, and polarization angles. The scattering process is much faster than the absorption/emission process. One of the most useful applications of light scattering spectroscopy is Raman spectroscopy.
Raman spectra contain vibrational information about the molecular species, thus allowing them to be uniquely identified with a spectral “fingerprint” by probing the molecule's vibration modes. Thus, Raman spectroscopy has potential applications in uniquely identifying chemical and biological agents. Unfortunately, Raman signatures are generally extremely weak and hence generally require large, delicate, bench-top spectrometers, with cooled detectors in order to detect the low level Raman signals emanating from the samples. This has limited the scope of Raman spectroscopy to generally being a research tool rather than an end-user diagnostics tool.
Surface Enhanced Raman Scattering (SERS) has emerged as an alternative to the traditional Raman scattering studies for species identification. SERS uses local, surface field enhancement to amplify the Raman scattering signal by many orders of magnitude (e.g., up to 10 to 12 orders of magnitude) as compared to conventional Raman spectroscopy. In SERS, a molecule or material to be identified is positioned in close proximity to one or more metallic nanoparticles, such as gold or silver nanoparticles. Due to proximity to the metal nanoparticle(s), the molecule to be identified experiences a greater electric field resulting from a resonant surface plasmon excitation, and consequently this enhanced evanescent field created near the surface of the nanoparticle amplifies the Raman vibrational mode and thus the Raman signal emanating from the molecule to be identified.
SERS studies in the laboratory using traditional Raman spectrometers have shown the capability to uniquely identify chemical agents and biological pathogens at very low concentrations approaching single molecule levels. SERS has therefore become an attractive modality for chemical and biological agent identification, such as for identifying pathogens. However, in order for a SERS spectrometer to become a practical field tool, a compact, light, rugged, and high performance spectrometer is needed.